Changing subcellular localization of nuclear transport factors during human spermatogenesis

Spermatogenesis requires progressive changes in gene expression mediated by hormonal and local factors. Regulated macromolecular movement between nuclear and cytoplasmic compartments enables these essential responses to changing extracellular cues, and dynamic production of the nucleocytoplasmic transporters and importin proteins, throughout gametogenesis in rodents implicates them as key mediators of germline differentiation. We examined normal adult human testis expression profiles of six importins plus five additional proteins involved in nucleocytoplasmic transport. Although most were detected in the nucleus during germline differentiation, importin alpha 4 was exclusively observed in Sertoli and germ cell cytoplasm. Many proteins were present in round spermatid nuclei (importins alpha 1, alpha 3, beta 1, beta 3; exportin-1, Nup62, Ran, RanBP1, RCC1), and remarkable intense nuclear and/or nuclear-associated signals were detected for importin alpha 1, importin alpha 3 and Nup62 in spermatocytes. This study identifies conserved aspects of nucleocytoplasmic transport during spermatogenesis and extends our knowledge of the dynamic presence of these proteins, which indicates that they contribute to germ cell-specific cargo trafficking and potentially to other functions during human spermatogenesis. We also demonstrate for the first time that importin alpha 3 is nuclear in spermatocytes, when exportin-1 is cytoplasmic, suggesting that nuclear transport is altered during meiosis.